An atlas of the distribution of NADPH-diaphorase in the brain of the highly derived swordtail fish Xiphophorus helleri (Atherinoformes:Teleostei).

The distribution of nicotinamide adenine dinucleotide phosphate-diaphorase (or -dehydrogenase, NADPHDH, a marker for nitric oxide synthase, NOS) positive neurons was demonstrated histochemically in the central nervous system of the swordtail fish Xiphophorus helleri, a highly derived telost of the atherinomorph outgroup. All nuclei of the swordtail fish, comprising almost all nuclei that have been described so far regarding the brains of telosts in general, were investigated. The results obtained clearly indicate, that NADPHDH positive neurons in the swordtail fish mostly are restricted to evolutionary primitive brain areas such as to most dorsomedial, dorsocentral and ventral portions of the telencephalon, to the preoptic area and to some thalamic, hypothalamic and pretectal nuclei of the diencephalon including the pituitary and the pineal organ, to the nuclei of the mesencephalic and myelencephalic cranial nerves, to the myelencephalic reticular formation and to the Mauthner cells. Some positive neurons were also observed within the mesencephalon, including the cerebellar body and its valve. Among non-neuronal cells, especially ependymocytes were strongly stained. It is the particular goal of the present study to provide a complete account on NADPHDH in the brain of a fish species, since, firstly, the NADPHDH/NOS system becomes increasingly important with regard to the understanding of the molecular basis of memory formation, and, secondly, fishes generally are more and more intensively studied concerning neurobiological approaches.